1. Field of the Invention
The present invention relates to an amplifier which outputs a predetermined voltage.
2. Description of the Related Art
Japanese Patent Application Publication No. 2008-293409 discloses a prior art amplifier circuit which generates a predetermined voltage.
Further, FIG. 13 shows another prior art amplifier which comprises a first amplifier including N-type field-effect transistors (hereinafter, N-type transistor) M1 to M3 and P-type field-effect transistors (hereinafter, P-type transistor) M4 and M5 and a second amplifier which comprises a P-type transistor M6 and resistances R1 and R2.
The N-type transistors M1, M2 of the first amplifier are input transistors whose substrate gates are connected to their respective sources. The sources thereof are connected to each other and the N-type transistor M3 is connected between a connecting point of the sources and a ground terminal GND. The N-type transistor M1 is connected to a reference voltage line Vref at its gate while the N-type transistor M2 is connected to a connecting point of the resistances R1, R2 at its gate.
The substrate gate and source of the N-type transistor M3 are connected to the ground terminal GND, a gate thereof is connected to the reference voltage line Vref and a drain thereof is connected to the sources of the N-type transistors M1, M2, to constitute a constant current source.
The P-type transistors M4, M5 are connected to a power supply voltage line Vcc at their substrate gates and sources and to drains of the N-type transistors M1, M2 at their drains. The gates of the P-type transistors M4, M5 are connected to each other and their connecting point is connected to the drain of the P-type transistor M5, thereby constituting a current mirror circuit.
The P-type transistor M6 of the second amplifier is connected to the power supply voltage line Vcc at its substrate gate and source and to an output Vout of the resistance R1 at its drain. It is also connected at its gate to a connecting point of the drains of the P-type transistor M4 and the N-type transistor M1. The other end of the resistance R1 is connected to the ground terminal GND via the resistance R2. Thus, the P-type transistor M6 of the second amplifier and the resistances R1, R2 constitute a feedback loop relative to the first amplifier.
In the circuit above, since the P-type transistors M4, M5 constitute a current mirror circuit, the same current flows in the N-type transistors M1, M2 and gate potentials thereof coincide with each other. Therefore, the following equation is satisfied:Vout=(R1+R2)/R2*Vref  (1)For example, when Vref=0.5V (not shown), R1=4,000 KΩ, and R2=2,000 KΩ, Vout=1.5V. In this case both of the minimal operation voltage and the output voltage Vout are 1.5V.
In order to operate the amplifier at a low voltage, both the operation voltage and output voltage Vout need be low. If the resistance R1 is changed to 3,000 KΩ, 2,000 KΩ, 1,000 KΩ in order while the resistance R2 is fixed at 2,000 KΩ, the output voltage Vout is assumed to change to 1.25V, 1.0V, 0.75V. However, at the resistance R1 being 1,000 K Ω or less, the output voltage Vout may sharply rise beyond a designed value as shown in FIG. 14.
This occurs because the minimal operation voltage of the first amplifier and that of the second amplifier differ in magnitude. Their minimal operation voltages MV1, MV2 are expressed by the following equations (2), (3), respectively.MV1 of the first amplifier=VdsM4+VdsM1+VdsM3  (2)MV2 of the second amplifier=VdsM6+Vref+(R2/R1)*Vref  (3)
Due to use of the same P-type transistor, the first terms of the equation (2), (3) are the same value so that the magnitude of the minimal operation voltages of the first and second amplifiers is determined by the magnitude of the sums of the second and third items.
In more detail, at R1 being 3,000 KΩ or 4,000 KΩ, or at R2/R1 being larger than 1 as shown in FIG. 14, the minimal operation voltage of the first amplifier is smaller than that of the second amplifier. After an output 200 of the first amplifier is stabled, the second amplifier starts operating. However, at R1 being 1,000 K, or at R2/R1 being smaller than 1, the minimal operation voltage of the first amplifier is larger than that of the second amplifier. Because of this, the output 200 becomes inconstant in a voltage range beyond the threshold voltage of the P-type transistor M6 before the operation of the first amplifier, disabling control over the P-type transistor M6 so that the P-type transistor M6 is turned on.
Furthermore, referring to FIG. 15, while the power supply voltage Vcc is low, either of the first and second amplifiers does not operate and the output 200 of the first amplifier is at a low level. With an increase in the power supply voltage Vcc, the second amplifier alone starts operating. The output 200 remains at a low level since the first amplifier is not in operation. When the P-type transistor M6 is brought into an ON-state, the output voltage Vout rises along with an increase in the power supply voltage Vcc (Vout=Vcc). With a further increase in the power supply voltage Vcc, the first amplifier starts operating with the second amplifier and outputs a voltage V200 to the output 200. This can control the P-type transistor M6 to make the output voltage Vout stable at a constant voltage. However, at R1 being 1,000 KΩ, since the first amplifier starts operating after the output voltage Vout sharply rises to about 1V, the output voltage Vout becomes stabled after sharply exceeding a designed value (0.75V).